1. Field of the Invention
The present invention relates to a plasma reactor, specifically, to a method and apparatus used in the manufacture of integrated circuits and other electronic devices. More particularly, the invention relates to modifying and uniformly distributing the gas concentration of a meta-stable or atomic species over a wafer in a downstream plasma reactor.
2. Description of Related Art
Plasma-based reactions have become increasingly important to the semiconductor industry, providing for precisely controlled thin-film depositions, thin film etching, and surface treatment such as cleaning. In a deposition process, thin films are applied to semiconductor wafers; whereas, an etching process is generally used in semiconductor manufacture to remove exposed portions of the deposited film for the purpose of patterning the film. One possible method for depositing films, such as a nitride film, is a remote plasma technique. In this method, a plasma is generated at a location which is separate from the wafer. Unlike non-remote plasma processes, downstream plasma processing allows for non-wet chemical processing while eliminating plasma-induced device damage. The plasma products are allowed to flow over the wafer. In this manner, the wafer is not subjected to ion or electron bombardment, or the high heat loads typical of in-situ plasma systems. The plasma source is also equally appropriate for etching and cleaning as it is for depositing.
Candidates for downstream processing are those reactions initiated by atomic species or molecular fragments that can be generated within an active (glowing) plasma. Downstream processing generates a chemical reaction between reactive gas effluents flowing from the plasma source and the materials on the wafer. Downstream reactions are driven by the concentration and flow speed of the reactant flux to the wafer surface, the reaction rate constant, and the removal of reaction products from the reaction site. One difficulty arises, however, when a non--excited gas is required to be injected into the chamber concurrently with an excited gas. Typically, two independent chamber input ports are needed to supply both gases. The introduction of gases from two separate ports complicates the distribution of the gas mixture over the wafer surface leading to film non-uniformity.
The remote plasma enhanced chemical vapor deposition (PECVD) process affords greater control over the thin-film chemistry than the conventional PECVD process by restricting plasma excitation to a subset of the process gases, and thereby reducing the number of possible reaction pathways. The physical arrangement of a remote PECVD chamber is designed to make the process flow sequential or serial, rather than parallel as in a conventional or direct PECVD processes. A description of a remote PECVD process can be found in J. A. Theil, et al., "EFFECTS OF NH3AND N2SOURCE GASES AND PLASMA EXCITATION FREQUENCIES ON THE REACTION CHEMISTRY FOR Si3N4THIN-FILM GROWTH BY REMOTE PLASMA-ENHANCED CHEMICAL-VAPOR DEPOSITION", J. Vac. Sci. Technology, A 10(4), July/August 1992, pp. 719-727.
Typically, a remote PECVD deposition process consists of the following steps: a) RF excitation of a first gas or gas mixture; b) transport of the excited species out of the plasma region into a chamber; c) introduction of a second gas over the substrate surface; and d) a CVD reaction at a substrate supported within the chamber to generate a thin dielectric film. For example, if a thin film silicon nitride were desired, the first gas would contain nitrogen, and the second gas would include a silicon containing gas such as Silane, SiH4.
FIG. 1 is a schematic representation of a prior art remote PECVD chamber 10. Importantly, these remote PECVD chambers provide for RF coils 12 surrounding a tube 14, typically a pyrex tube, to inductively excite a gas delivered at the top 16 of reactor 10. The excited gas is then transported into chamber 20 through input port 18. The gas disperses within chamber 20 and reacts with substrate 22 which is supported on pedestal 24. A similar PECVD chamber has been previously discussed by D.V. Tsu, et al., in "LOCAL ATOMIC STRUCTURE IN THIN FILMS OF SILICON NITRIDE AND SILICON DIIMIDE PRODUCED BY REMOTE PLASMA-ENHANCED CHEMICAL-VAPOR DEPOSITION," Physical Review B, Volume 33, Number 10, May 15, 1996, p. 7070. In the Tsu invention, a second gas is delivered through a feed-through tube to a gas dispersal ring that is placed over the substrate. This second gas is typically delivered through a second input port, shown in FIG. 1 as covered by plate 19. Although the second gas delivery apparatus, i.e., gas dispersal ring, is not common to all prior art remote PECVD chambers, it nevertheless further contributes to gas concentration non-uniformity at the wafer surface. It also represents a current prior art method for introducing a second, unexcited gas into the chamber.
Process uniformity has been previously attempted by establishing the flow dynamics that help control a uniform species distribution across the reacting surface. An inherent disadvantage of a remote plasma system, however, is the lack of acceptable process uniformity of the gas distribution at the wafer or substrate level. Since the active gases created by the plasma are delivered to the process chamber and not created in it, the distribution of gases inside the chamber is very difficult to control due to unwanted reactions on the chamber surfaces which consume the active gases. If the reactive gas flows in from the side of the reactor, with respect to the wafer, the concentration will be high in the center of the wafer and low at the edges.
In typical (non-remote) Chemical Vapor Deposition (CVD) or Plasma Enhanced Chemical Vapor Deposition (PECVD) reactors, a showerhead is used to make the gas distribution uniform over the wafer. This strategy is not suitable for downstream reactors because the active gasses would have to flow past the baffle and faceplate holes which make up the showerhead. The showerhead elements also have the effect of destroying the active species needed for remote processing.
In the case of an active atomic species being generated in the remote plasma, the showerhead elements promote recombination. In the case where the remote plasma generates a meta-stable species, the showerhead elements promote "quenching" or deactivation of the species.
The distribution of gas during plasma processing can also be affected by the introduction of a second gas. Concurrent injection of two gases (or gas mixtures) is typically performed by introducing the second (unexcited) gas through a separate input port into the chamber. However, this second injection will alter the uniform distribution of the excited gas, requiring that at least two separate distribution normalization systems or processes be employed. An apparatus and method capable of concurrent injection through the same input port would allow for unique advantages in the distribution normalization of the gas mixture, and eliminate the need for a second input to the chamber.
Additionally, concurrently providing two independent gases to the chamber through the same plasma confinement tube, one gas of which is excited by the plasma source while the other is isolated from the RF inductive and infrared radiated energies, facilitates the simultaneous introduction of diverse gas mixtures within the process chamber. Also, one may introduce a single gas within the chamber causing it to have an excited component and a non-excited component.
A supersonic CVD gas jet source for deposition of thin films has been developed in U.S. Pat. No. 5,256,205, issued to J. Schmitt, et al., entitled "MICROWAVE PLASMA ASSISTED SUPERSONIC GAS JET DEPOSITION OF THIN FILM MATERIALS." This source has been used to produce a high dielectric constant for thin film semiconductor applications, e.g., Si3N4. However, this source does not provide for simultaneous delivery of active and molecular (non-activated or dissociated) gas species.
Also, it is advantageous to have a source that is not dependent upon the supersonic flow of the material to be deposited on the semiconductor substrate. It is beneficial to have material transport and growth not made dependent only upon diffusion kinetics in high vacuum, e.g., 300 mTorr. Less process gas flow at lower chamber pressures will reduce chamber design and construction constraints.
Lastly, prior art sources of concurrent gas delivery systems are limited to an area at the wafer surface about 1 cm to 2 cm in diameter, restricted mainly by the Laval nozzle dimension, and require translation of the substrate in a complex motion in front of the nozzle in order to achieve deposition over an entire substrate of any dimension greater than approximately 1 cm.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an apparatus and method for injecting gases and enhancing the uniformity of the active species in a remote downstream plasma reactor.
It is another object of the present invention to provide an apparatus and method for tailoring the gas concentration distribution of a remote PECVD process for wafer-edge or wafer-center concentrations.
A further object of the invention is to provide an apparatus and method for varying the gas concentration distribution across a wafer in a remote PECVD process.
Another object of the invention is to provide a remote PECVD source that can deposit uniform thin films across the entire field of the substrate with no movement or translation of the substrate or source.
It is yet another object of the present invention to provide a remote PECVD process that minimizes the loss of reactive species.
A further object of the present invention is to provide an apparatus and method to introduce gases into the chamber concurrently, through the plasma tube, having one gas excited by the plasma tube energies and the other remaining unexcited and shielded from the plasma tube energies.
Still other advantages of the invention will in part be obvious and will in part be apparent from the specification.